CloneCommand#setCallback should return 'this'

The other methods in this class follow the builder pattern, and
return 'this', allowing multiple method calls to be chained in a
single statement.

Update the setCallback method to do the same.

Change-Id: I0366d28bf66ba47f08ee7eee636d613c9fe079f5
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/api/CloneCommand.java

@@ -611,10 +611,12 @@ public class CloneCommand extends TransportCommand<CloneCommand, Git> {
 	 *
 	 * @param callback
 	 *            the callback
+	 * @return {@code this}
 	 * @since 4.8
 	 */
-	public void setCallback(Callback callback) {
+	public CloneCommand setCallback(Callback callback) {
 		this.callback = callback;
+		return this;
 	}
 
 	private static void validateDirs(File directory, File gitDir, boolean bare)